Nickel base superalloys are widely used in gas turbine engines. One application is for turbine disks. The property requirements for disk materials have increased with the general progression in engine performance. Early engines used easily forged steel and steel derivative alloys for disk materials. These were soon supplanted by the first generation nickel base superalloys such as Waspaloy which were capable of being forged, albeit often with some difficulty.
Nickel base superalloys derive much of their strength from the gamma prime phase. The trend in nickel base superalloy development has been towards increasing the gamma prime volume fraction for increased strength. The Waspaloy alloy used in the early engine disks contains about 25% by volume of the gamma prime phase whereas more recently developed disk alloys contain about 40-70% of this phase. The increase in the volume fraction of gamma prime phase reduces the forgeability. Waspaloy material can be forged from cast ingot starting stock but the later developed stronger disk materials cannot be reliably forged and require the use of more expensive powder metallurgy techniques to produce a disk preform which can be forged and then economically machined to final dimensions. One such powder metallurgy process which has met with substantial success for the production of engine disks is that described in U.S. Pat. Nos. 3,529,503 and 4,081,295. This process has proved highly successful with powder metallurgy starting materials but less successful with cast starting materials.
Other patents relating to the forging of disk material include U.S. Pat. Nos. 3,802,938; 3,975,219; 4,110,131, 4,574,015 and 4,579,602. This invention is in some respects an extension of the process of U.S. Pat. No. 4,574,015 patent which is incorporated herein by reference.
In summary therefore, the trend towards high strength disk materials has resulted in processing difficulties which have been resolved only through recourse to expensive powder metallurgy techniques.
It is an object of the present invention to describe a method through which cast high strength superalloy materials may be readily forged.
Another object of the present invention is to provide a method for producing forging preforms from cast superalloy materials which contain in excess of about 40% by volume of the gamma prime phase and which would otherwise be unforgeable.
A further object is to disclose a combined heat treatment, extrusion and forging process which will produce superalloy articles with void free fully recrystallized microstructures having a uniform fine grain size.
It is yet another object of the invention to provide a highly forgeable nickel base superalloy preform having an overaged gamma prime morphology with an average gamma prime size in excess of about 2 microns and a fully recrystallized microstructure.